Pulsed High Magnetic Fields Research Articles

NMR signal averaging in 62 T pulsed fields

Nuclear Magnetic Resonance (NMR) experiments in pulsed high magnetic fields up to 62 T at the Dresden High Magnetic Field Laboratory ( Hochfeld-Magnetlabor Dresden) are reported. The time dependence of the magnetic field is investigated by observing various free induction decays (FIDs) in the vicinity of the maximum of the field pulse. By analyzing each FID’s phase and its evolution with time the magnetic field’s time dependence can be determined with high precision. Assuming a quadratic or cubic dependence on time near the field maximum its confidence is found to be better than ±0.03 ppm at low fields and ±0.8 ppm near 62 T. In turn, the thus obtained time dependence of the field can be used to demodulate and phase-correct all FIDs so that they appear phase-locked to each other. As a consequence signal averaging is possible. The increase in signal-to-noise ratio is found to be close to that expected theoretically. This shows that the intrinsic time dependence of the pulsed fields can be removed so that the NMR signals appear to be taken at rather stable static field. This opens up the possibility of performing precise shift measurements and signal averaging also of unknown, weak signals if a reference signal is measured during the same field pulse with a double-resonance probe.

ELECTRONIC PROPERTIES OF GRAPHENE, FEW-LAYER GRAPHENE, AND BULK GRAPHITE UNDER VERY HIGH MAGNETIC FIELD

In the present work, we report on the magneto-transport properties of graphitic based materials (graphene, few-layer graphene, and bulk graphite) in very high magnetic field. Quantum Hall Effect (QHE) has been studied in graphitic systems in very high pulsed magnetic field (up to B = 57 T ) and at low temperature (≤ 4 K). Graphene sample shows well-defined Hall resistance plateaus at filling factors v = 2,6,10, etc. Few-layer graphene systems display clear signatures of standard and unconventional QHE. Magneto-transport studies on bulk highly oriented pyrolytic graphite show a charge density wave transition at strong enough magnetic field as well as Hall coefficient sign reversal.

Multi-frame acquisition scheme for efficient energy-dispersive X-ray magnetic circular dichroism in pulsed high magnetic fields at the FeK-edge

Using a fast silicon strip detector, a multi-frame acquisition scheme was implemented to perform energy-dispersive X-ray magnetic circular dichroism at the iron K-edge in pulsed high magnetic fields. The acquisition scheme makes use of the entire field pulse. The quality of the signal obtained from samples of ferrimagnetic erbium iron garnet allows for quantitative evaluation of the signal amplitude. Below the compensation point, two successive field-induced phase transitions and the reversal of the net magnetization of the iron sublattices in the intermediate phase were observed.

25 Tesla pulsed-high-magnetic-field system for soft X-ray spectroscopy

We have developed a 25 T pulsed high magnetic field system for soft X-ray Magnetic Circular Dichroism: XMCD. The ultra-high vacuum chamber with a pulse magnet coil is installed. By using a newly developed bipolar capacitor bank, the XMCD of paramagnetic Gd 2O 3 at the M 5 and the M 4 edges was clearly observed at low temperatures. The present system is capable of measuring XMCD of field induced moments in various compounds including paramagnets and antiferromagnets.

Effect of non-magnetic doping on multi-step magnetization behavior of Dy 50− xAl xAg 50

The electrical transport and magnetization measurements have been carried out on Al-doped polycrystalline intermetallic compounds Dy 50− x Al x Ag 50 ( x = 0, 0.3, 0.6, 1.2, 1.8) in a pulsed high magnetic field, in which multi-step magnetization is observed. Partial substitute of non-magnetic Al 3+ for Dy 3+ ions in the compounds increases the critical magnetic fields and the relative area of the magnetic hysteresis loop, which result from the pinning effect, lattice distortion, the change of coupling strength and dilution effect related to the Al 3+ doping.The experimental results indicate that non-magnetic Al 3+ ions and induced amorphous phase can pin the rotation and/or growth of magnetic domains, thus, the critical magnetic field can be enhanced by doping non-magnetic ions in the magnetic materials, especially in the permanent magnet materials.

Magnetization and Giant Negative Magneto-Resistance in Nano-Scale Ni<sub>3-δ</sub>Mn<sub>1+δ</sub> in Pulsed Magnetic Fields up to 25 T

Magneto-transport phenomena of slightly Mn-rich sample in the host Ni3-Mn system were investigated on giant-negative magneto-resistance of 15% in pulsed high magnetic fields up to 25 T. We prepared nano-scale crystals of Nin (0≤≤0.05) fabricated by rapid quenching method with a mono-role and by optimum annealing to obtain the maximum magneto-resistance effect in the longitudinal configuration of sample currents and magnetic fields. The negative magneto-resistance observed in this study was interpreted as the scattering decrease of conduction electrons by anti-parallel spin pairs composed of Mn-Mn deviated from the stoichiometry. This physical mechanism also reflects the coherent length change of itinerant electrons, caused by the scattering by the comparable short-range spin disorder of Mn-Mn pairs in the nano-scale crystal grain.

Mechanical design of a dual-cryostat instrument for a high-field pulsed magnet

The engineering of a dual-cryostat for a pulsed-magnet instrument at the Advanced Photon Source is presented. The dual-cryostat independently cools the magnet coil (using liquid-nitrogen) and the sample (using a closed-cycle refrigerator). Liquid-nitrogen cooling may allow a repetition rate of a few minutes for peak fields near 30 T. The system is unique in that the liquid-nitrogen cryostat incorporates a double-funnel vacuum tube passing through the solenoid's bore in order to preserve the entire angular range allowed by the magnet bore for scattering studies. Second, the use of a separate refrigerator for the sample allows precise positioning of samples in the bore while minimizing magnet vibrations propagating to the sample during pulsed-field generation.

Fabricating Co Doped ZnO Nanocrystallines by Hydrothermal Method with High Pulsed Magnetic Field

Co-doped ZnO nanocrystallines were fabricated by hydrothermal method with high pulsed magnetic field. As a new preparation method, more refined grains and diluted magnetic semiconductors (DMSs) with better performance could be obtained by the hydrothermal synthesis process under pulsed magnetic field. The samples were tested by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Vibrating Sample Magnetometer (VSM) and Raman scattering spectrum. The results show that the sample prepared under pulsed magnetic field has wurtzite structure with tiny crystal lattices changes. Doping Co ions into ZnO crystal lattice is also improved. Curie temperature of 2% Co doped ZnO nanocrystallines synthesized with 4T pulsed magnetic field is higher than that for without magnetic field process, and very close to the room temperature.

Magnetic field strength and orientation effects on Co-Fe discontinuous multilayers close to percolation

Magnetization and magnetoresistance in function of the magnitude and orientation of applied magnetic field were studied in Co-Fe discontinuous multilayers close to their structural percolation. The high pulsed magnetic fields up to 33 T were used in the 120--310 K temperature range. Comparison between longitudinal and transverse (with respect to the film plane) field configurations was made in the low-field and high-field regimes in order to clarify the nature of the measured negative magnetoresistance. Coexistence of two distinct magnetic fractions, superparamagnetic (SPM, consisting of small spherical Co-Fe granules) and superferromagnetic (SFM, by bigger Co-Fe clusters), was established in this system. These fractions were shown to have different relevance for the system magnetization and magnetotransport. While the magnetization is almost completely (up to $\ensuremath{\sim}97\mathrm{%}$) defined by the SFM contribution and practically independent of temperature (in this range), the magnetoresistance experiences a crossover from a regime dominated by Langevin correlations (suppressed with temperature) between neighbor SPM and SFM moments at low fields, to that dominated by spin scattering (enhanced with temperature) of charge carriers within SFM clusters at high fields. Also, the demagnetizing effects, sensitive to the field orientation, were found to essentially define the low-field behavior and characteristic crossover field.

Tunnel diode oscillator-based measurement of quantum oscillations amplitude in pulsed high magnetic fields: a quantitative field-dependent study

Tunnel diode oscillator-based technique has been used for quantum oscillations spectra determination of a quasi-two dimensional organic metal in pulsed high magnetic fields of up to 55 T. It is demonstrated that reliable field-dependent quantitative data can be obtained in the case of complex oscillatory spectra provided adequate data processing is conducted.

Transport of laser accelerated proton beams and isochoric heating of matter

The acceleration of intense proton and ion beams by ultra-intense lasers has matured to a point where applications in basic research and technology are being developed. Crucial for harvesting the unmatched beam parameters driven by the relativistic electron sheath is the precise control of the beam. We report on recent experiments using the PHELIX laser at GSI, the VULCAN laser at RAL and the TRIDENT laser at LANL to control and use laser accelerated proton beams for applications in high energy density research. We demonstrate efficient collimation of the proton beam using high field pulsed solenoid magnets, a prerequisite to capture and transport the beam for applications. Furthermore we report on two campaigns to use intense, short proton bunches to isochorically heat solid targets up to the warm dense matter state. The temporal profile of the proton beam allows for rapid heating of the target, much faster than the hydrodynamic response time thereby creating a strongly coupled plasma at solid density. The target parameters are then probed by X-ray Thomson scattering (XRTS) to reveal the density and temperature of the heated volume. This combination of two powerful techniques developed during the past few years allows for the generation and investigation of macroscopic samples of matter in states present in giant planets or the interior of the earth.

Abnormal magnetization and field-induced transition in (La 0.73Bi 0.27) 0.67Ca 0.33MnO 3

The magnetic field dependence of magnetization of Bi doped manganites (La 1− x Bi x ) 0.67Ca 0.33MnO 3 ( x=0.27) was investigated at different temperatures with a pulsed high magnetic field. A metamagnetic transition was observed in the magnetization measurement, which revealed the coexistence of charge ordering (CO) and ferromagnetic (FM) phases. With decreasing magnetic field, the field-induced FM phases remained stable even when the magnetic field decreased to zero. This result suggests that ferromagnetic interactions are enhanced due to the effect of the pulsed high magnetic field, which makes the doped manganites a good system for magnetoresistance materials.

Characteristics of High Strength and High Conductivity Cu-Nb Micro-Composites

We have fabricated Cu-Nb composites with high strength and high conductivity by bundling and drawing process for their application in high field pulsed magnets. Several kinds of composites are designed for investigation of the filament-size and interface effect on the strength and conductivity. For Cu-Nb micro-composites with a section of 5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> composed of a Cu matrix embedded 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> continuous Nb nano-fibers with a diameter of 100 nm, our results have shown that the ultimate tensile strength reaches 1 GPa while the electrical conductivity is higher than 70% IACS at room temperature. Since the size of Nb filaments is already on a nanometer scale, the size effect on microstructure and property of Cu-Nb composites has been studied.

Far-Infrared Ferromagnetic Resonance of Magnetic Garnet for High Frequency Electromagnetic Sensor

For the sensor probe of high frequency magnetic field in GHz region, films were prepared by liquid phase epitaxy technique and ferromagnetic resonances were investigated in high frequency region. Magneto-optical (MO) effect of magnetic garnet was utilized for the imaging sensor of the magnetic field distribution. However, for the application to the characterization of high frequency magnetic field in the GHz frequency region, usual MO measurements are difficult to detect the magnetic filed as the decreasing of the permeability of magnetic garnet. Magnetic resonance is an effective way to induce magnetic moment and enhance the magnetooptical effect in high frequency region of GHz order. In order to understand the possibility of magnetic field sensor of high frequency in detail, far-infrared magnetic resonances were measured at the high frequencies up to 315 GHz in pulsed high magnetic field. Taking account into sensitivity of MO signals and magnetic resonance intensities, magnetooptical method using a garnet film is effective at the frequencies below 100 GHz.

A Software Approach for Real-Time Control of Operation and Experiments at the WHMFC Pulsed High Magnetic Field Facility

A newly developed Control Software System (CSS) designed to control and monitor the operation of the magnets and the experiments at the Wuhan High Magnetic Field Center (WHMFC) pulsed high magnetic field facility is described. The CSS implements the functions of status evaluation and experiment process control in real time as well as the functions of system structure configuration and data management off-line. Using software architecture based on component-oriented technology, scenarios about control flow rebuilding, interfacing with local control hardware, synchronous triggering and transfer of the results of measurements are discussed in detail. Hardware simulator design and human machine interface of the CSS are described as well. The test results show that the entire operation process is perfectly handled by the CSS which provides an integrated platform for scientific experiments and research.

Design of Multipulse Power Supply for Small Pulsed High Magnetic Field Device

Three small pulsed high magnet field devices (SPHMF) which can continuously operate 20 times in rated power with different maximum magnetic field intensity from 10 T~40 T were developed in Wuhan High Magnet Field Center(WHMFC). The SPHMF systems are used to investigate the sterilization of fruit juice, magnetization of permanent magnet for wind power generator, some industrial applications and so on. The SPHMF consists of pulsed capacitor power supply, charger, control & measurement system, cryogenic system and pulsed magnet. These systems can accommodate sample room size from ¿12 mm to ¿18 mm and hold sample room in indoor temperature 300 K for different scientific researches and high magnetic field industrial applications. In this paper, power supply, magnet, control system and cryogenic system are presented. The total experiment results of SPHMF show that the devices are feasible, steady and reliable.

Development of an Optical Digitized Local Control and Measurement System Applied to Pulsed High Magnetic Field Facility

A reliable controlling and measuring system in a pulsed high magnetic field facility is important for efficient operation. To control and monitor the status of switches, relays and thyristors, an optical digitized local controlling and measuring system (LCMS) was applied in Wuhan High Magnetic Field. On the basis of a digital signal processor (DSP) with specific type of TMS320F2812, LCMS receives and executes the commands including setting parameters, charging, dumping and discharging issued from the Human-Machine Interface (HMI) at the control room. Meanwhile, a fiber optic transmission system is needed to provide galvanic isolation. The discharge current and voltage of capacitor bank are measured by Rogowski coil and resistance divider respectively. The whole system test shows that the designed system is effective and reliable.

Prototype Test and Manufacture of a Modular 12.5 MJ Capacitive Pulsed Power Supply

A new Pulsed High Magnetic Field Facility (PHMFF) is under construction at the Huazhong University of Science and Technology (HUST) in Wuhan, China. In order to provide a wide spectrum range, a modular capacitive pulsed power supply is designed to energize 12.5 MJ at 25 kV and a large variety of coils are designed for magnetic fields in the 50-80 T with pulse duration 15-200 ms, respectively. In order to energize multi-coil systems, the 12.5 MJ power supply is divided into 11 independent 1 MJ modules with a short circuit current of 44.7 kA each and 2 independent 0.75 MJ modules for 61.2 kA each. Each module is provided with its own protection inductor, thyristor switch, crowbar circuit, dump circuit, polarity-changing circuit, charging unit and switchgear. The current pulses can be modified in shape, amplitude and duration by varying the charging voltage, the number of modules and the crowbar resistors. For verifying the design scheme and gaining experience in the construction and operation of pulsed power supply, a 1 MJ prototype that is used to energize pulsed magnets for 50-72 T fields with reversible polarity has been developed. In this paper, after the introduction of the system configuration with different module types and system settings, the design of both the 1 MJ and 0.75 MJ modules is described in detail. Finally, test results of the 1 MJ prototype are presented.

Evidence for Anisotropic Vortex Dynamics and Pauli Limitation in the Upper Critical Field of FeSe1-xTex

We have determined HC2(T) for FeSe1-xTex (x=0.52) single crystals using resistivity measurements at high static and pulsed magnetic field, as well as specific heat measurements up to 9T. We find that the significant anisotropy of the initial slope of HC2(T) determined from resistivity measurements, is not present when HC2 is determined from the specific heat results. This suggests that the thermodynamic upper critical field is almost isotropic, and that anisotropic vortex dynamics play a role. Further evidence of anisotropic vortex dynamics is found in the behaviour in pulsed field. We also find that Pauli limiting must be included in order to fit the temperature dependence of HC2, indicating probably higher effective mass in FeSe1-xTex than in other Fe superconductors.

Effects of La, Nd and Sm substitution of Sr in Sr 2CrReO 6 on the structural, magnetic and transport properties

A detailed study of Strontium substitution (up to 25% at most) with Lanthanide (La, Nd and Sm) in the Sr 2CrReO 6 double perovskite has been carried out by means of neutron powder diffraction (NPD), scanning electron microscopy (SEM), low magnetic field and high pulsed magnetic field magnetometry as well as transport measurements. In the present manuscript we will focus on the consequences of such substitution on structural, electronic and magnetic properties. The presented results show the lack of expected enhancement of the Curie temperature by the Lanthanide addition. This puts doubts on the possibility of electron doping in the Sr 2CrReO 6 system via the standard solid-state reaction growth techniques with rare-earth doping, commonly used for other double perovskite compounds.